2020 H2 Chem P1 Q19 — Alkane Solution

The answer is D (2 only).

• Statement 1 is incorrect: in the initiation step, UV light breaks the weakest bond. From the Data Booklet, C-F = 485 \(\text{kJ mol}^{-1}\) and C-Cl = 340 \(\text{kJ mol}^{-1}\), so the C-Cl bond breaks, giving Cl\(\bullet\) and \(\bullet\text{CClF}_2\), not F\(\bullet\) and \(\bullet\text{CCl}_2\text{F}\).
• Statement 2 is correct: X\(\bullet\) is used up in the first step and regenerated in the second. Adding 2 \(\times\) the first step to the second step gives the overall reaction \(2\text{O}_3 \rightarrow 3\text{O}_2\). X\(\bullet\) speeds up the breakdown of ozone without being used up overall, so it acts as a catalyst.
• Statement 3 is incorrect: in a termination step two radicals combine to form a molecule (for example \(2\text{Cl}\bullet \rightarrow \text{Cl}_2\)), so halogen radicals are removed, not formed. They are formed in the initiation step.

• A is wrong: it includes statements 1 and 3, which are incorrect.
• B is wrong: statement 1 is incorrect, and statement 2 is left out.
• C is wrong: it includes statement 3, which is incorrect.